Prior art has been directed to providing a two tier venetian blind with an upper tier and a lower tier wherein the slats may be adjusted to different angles in each tier. This allows the slats in the lower tier to be closed for privacy while the upper slats are open for light, or vice versa for shade from the closed upper tier and ventilation from the open bottom tier.
U.S. Pat. No. 4,621,672 to Hsu discloses a mechanism that will provide such independent control of upper and lower tiers of a venetian blind using a single control rod. The Hsu device does not address the problem of the control cords for the lower slats hanging when the upper tier of the blind is raised.
The upper and lower tiers require separate control cords so there are two adjacent control cords on the front of the blind and two on the back leading down from the blind control mechanism at the top of the blind. The upper control cords only reach to the bottom of the upper tier of slats, and are joined front to back by cross cords that act to tilt the slats when the control cords are moved--as the front cord goes up the back cord goes down and the cross cords, and therefore the slats, are tilted accordingly. The lower control cords are not connected by cross cords until they reach the bottom tier of slats. Thus when the bottom tier slats are tilted the upper tier slats do not move.
The blind acts normally during the first stage of raising with the lower front and back control cords being gathered by their attachment to the cross cords. Once the lower tier has been raised however, there are no more cross cords attaching those front and back lower control cords and so they hang in a loop when the upper tier slats are raised. In the Hsu device those cords merely hang, being then unsightly and presenting a possible hazard.
U.S. Pat. No. 5,485,874 to Whitmore addresses this hanging lower control cord problem by providing various mechanisms such as washers, cord loops and bands, by which the upper and lower control cords on each side of the blind are held together when the upper tier of the blind is raised. In one embodiment he also provides apertures on the front and back side of each slat through which the front and back lower control cords pass.
A simple cord arrangement solving this problem that involves no modification to the slats of an existing blind would be beneficial. Costs for providing a two tier blind would be reduced if only the blind control mechanism at the top of the blind required modification to provide a two tier blind.